Contact the cruise line you are interested in, and ask specifically what documentation will be required for your friend and the dog. If you are disembarking in a foreign port, you need to make sure what the regulations are there, and if the dog can go ashore.

I know of someone with limited sight that has cruised extensively with a service dog. The cruise line normally will go out of their way to make accomodations. On one cruise, the crew actually made a dog house and brought in a square of grass for the dog to have on the cabin balcony! The dog actually lived "inside" with it's owner and was trained to use the "indoor" facilities (special paper/liner placed in the shower). She had a jeweled collar for formal night, and colorful bandanas for island night! She rested under the table at our feet during dinner in the dining the room on the Millennium. She attended muster drill with her own life jacket. She guided her human back to the cabin, up and down elevators, all over the ship. It was amazing.